A growth substrate ball based on a biochar-soil composite structure and a method for preparing the same

Abstract

This invention belongs to the field of soil improvement and biochar utilization, and provides a growth substrate sphere based on a biochar-soil composite structure and its preparation method. This invention employs a quaternary composite design of fine soil particles, biochar, biochar-basalt core-shell composite particles, and alginate solids. A tannic acid-ferric chloride coordination composite layer connects the biochar core and the basalt shell layer. Then, calcium ion crosslinking of alginate solidifies all components into spheres. This achieves a balance between pressure resistance and abrasion resistance with air permeability and water retention, a balance between ion slow release and dimensional stability, and a balance between sphere processingability and service stability. It solves the triple contradictions of existing substrate materials between structural strength and pore connectivity, hydrophilic gel exchange function and dry / wet stability, and processing fluidity and interfacial bonding strength, and has broad application value in soil remediation and plant cultivation.

Description

Technical Field

[0001] This invention relates to the field of soil improvement and biochar utilization, specifically to a growth substrate sphere based on a biochar-soil composite structure and its preparation method. Background Technology

[0002] Soil degradation and the optimization of plant growth media performance are significant challenges in modern agriculture and ecological restoration. In applications such as saline-alkali land remediation, vegetation restoration of abandoned mining areas, and soilless cultivation in facility agriculture, stringent requirements are placed on the multidimensional performance of growth substrate materials. First, the substrate material needs good mechanical strength to withstand the mechanical stress during transportation, laying, and irrigation, preventing breakage and fine powder blockage of pores; simultaneously, it must maintain sufficient pore connectivity to achieve the aeration required for root respiration and the water retention capacity required for water transport. Second, the substrate material should possess nutrient slow-release and ion-exchange functions, providing the necessary nutrients for plant growth while buffering harmful ions in the soil solution. This is crucial for improving the rhizosphere microenvironment in saline-alkali and heavy metal-contaminated soils. Third, the substrate material should maintain dimensional and structural stability after repeated wet-dry cycles, avoiding pore structure damage and root injury caused by shrinkage and expansion. Furthermore, from an industrial production perspective, the molding and processing of substrate materials requires suitable rheological properties and a wide process window to reduce production costs and improve product quality stability. Meeting the above-mentioned multi-dimensional performance requirements is of great significance for improving soil remediation efficiency, expanding the application range of growth substrate materials, and promoting the advancement of ecological agriculture technology.

[0003] To address the aforementioned needs, existing technologies primarily employ simple mixing or granulation methods of biochar, soil particles, or mineral materials to prepare growth substrates. However, these methods generally suffer from performance conflicts that are difficult to balance. For example, Chinese patent application CN119351113B discloses a biochar-based soil conditioner and its preparation method, which involves simply mixing biochar with soil and adding a binder for granulation. However, this method has the following shortcomings: simply relying on compaction to increase particle strength significantly reduces porosity and aeration / water retention capacity, creating a contradiction between structural strength and pore function; binders are mostly organic polymers, whose hydrophilic swelling properties cause significant particle size changes during wet-dry cycles, making it difficult to maintain structural stability; while organic binders can improve rheological properties during the pelletizing stage, the interfacial bonding strength after curing is insufficient, and particles are prone to surface erosion and fine powder loss under hydraulic scouring. Therefore, developing a growth substrate pellet preparation technology that can simultaneously resolve multiple contradictions such as structural strength and pore function, ion exchange and dimensional stability, and pelletizing processability and service bonding strength is of significant practical importance. Summary of the Invention

[0004] The purpose of this invention is to provide a growth substrate sphere based on a biochar-soil composite structure and its preparation method, which solves three major problems of existing growth substrate materials: the difficulty in achieving both compressive strength and wear resistance due to structural densification and air permeability and water retention due to pore connectivity; the intrinsic conflict between alginate hydrophilic ionogels in terms of ion exchange slow release and dimensional stability during wet-dry cycles; and the mutual constraint between the requirements for low viscosity and wide processing window during the sphere formation stage and the interfacial bonding requirements for water erosion resistance and fine powder loss during the service stage.

[0005] This invention employs a quaternary synergistic design concept: "fine soil particle skeleton support + biochar porosity regulation + multi-scale reinforcement of biochar-basalt core-shell composite particles + alginate gel interfacial bonding and ion slow release." The biochar core provides micropores and specific surface area, the basalt shell provides an inorganic rigid framework, the coordination composite layer achieves strong core-shell interface connectivity, and the alginate gel network provides ion exchange sites, solidifying all components into a unified structure. This achieves synergistic optimization of mechanical strength and pore connectivity, ion slow release and dimensional stability, and spheroidal rheology and interfacial adhesion at multiple spatial scales, resulting in comprehensive performance that cannot be achieved by a single component or simple mixing, exhibiting a significant synergistic effect. The components form continuous mechanical transfer paths and multi-scale pore gradient structures through interfacial chemical bonding and spatial gradation, generating a nonlinear enhancement effect.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A growth substrate sphere based on a biochar-soil composite structure, wherein the growth substrate sphere is a spherical or near-spherical particle, the aspect ratio of the near-spherical particle is not greater than 1.5, and the aspect ratio is the ratio of the maximum diameter to the minimum diameter of the particle; the growth substrate sphere comprises the following solid components: Fine soil particles; Biochar; Biochar-basalt core-shell composite particles; Alginate solids, calculated as sodium alginate; The alginate solid is an alginate gel formed by calcium ion cross-linking and curing. The biochar-basalt core-shell composite particles include a biochar core, a basalt shell continuously covering the outer surface of the biochar core, and a coordination composite layer located between the biochar core and the basalt shell. The coordination composite layer is formed by tannic acid and ferric chloride hexahydrate; The basalt shell loading amount is 5wt% to 40wt%, and the shell loading amount is the mass percentage of the basalt shell mass to the total dry mass of the core-shell composite particles. The biomass carbon core, coordination composite layer, basalt shell, and silanized layer on the outer surface of the basalt shell constitute a multi-level interface reinforcement structure, which is used to improve the stability and determinism of carbon sequestration in the ecological restoration environment.

[0007] Furthermore, the biochar-basalt core-shell composite particles are prepared through the following steps: A1. Coordination Layer Construction: Hydrogen peroxide-treated biochar intermediates were dispersed in an aqueous tannic acid solution with a mass concentration of 0.1 g / L to 5 g / L and a solid-liquid mass ratio of 1:5 to 1:30. The mixture was stirred at 15°C to 40°C for 5 to 60 minutes. Subsequently, an aqueous solution of ferric chloride hexahydrate with a concentration of 0.01 mol / L to 0.2 mol / L was added. The amount of ferric chloride hexahydrate added was based on a molar ratio of Fe³⁺ to tannic acid of 0.1:1 to 2:1 and a molar mass of tannic acid of 1701.2 g / mol. The mixture was stirred at 15°C to 40°C for another 5 to 60 minutes to form biochar particles containing a coordination composite layer. A2. Shell introduction: Basalt powder is added to the system obtained in step A1, wherein the mass ratio of basalt powder to biochar is 0.1:1 to 2:1, and the mixture is stirred at 15°C to 40°C for 10 min to 120 min. A3. Silanization: The solid obtained in step A2 is filtered, washed with deionized water, and dried at 50°C to 80°C for 2 to 12 hours. It is then dispersed in a mixed solvent of ethanol and water for silanization. A silane coupling agent, 3-aminopropyltriethoxysilane, is added to the silanization reaction system at an amount of 0.1 wt% to 5 wt% based on the dry weight of the solid obtained in step A2. The volume ratio of ethanol to water is 1:9 to 9:1, and the ethanol is anhydrous ethanol or ethanol with a volume fraction of not less than 95 vol%. The liquid-solid mass ratio of the silanization reaction system is 5:1 to 30:1 based on the dry weight of the solid obtained in step A2. The pH value is adjusted to 5.0 to 8.0 by dropwise addition of a 1.0 mol / L hydrochloric acid aqueous solution or a 1.0 mol / L sodium hydroxide aqueous solution. The reaction temperature is 20°C to 50°C, the reaction time is 0.5 to 4 hours, and the reaction pressure is atmospheric pressure. A4. Post-processing: Filtration, washing with deionized water, and drying at 50°C to 80°C for 2 to 12 hours to obtain the biochar-basalt core-shell composite particles; A5. Quality control: The shell loading of the biochar-basalt core-shell composite particles is 5wt% to 40wt%, and the shell loading is the mass percentage of the basalt shell mass to the total dry weight of the biochar-basalt core-shell composite particles. The shell loading is determined by thermogravimetric analysis or ash content determination.

[0008] Furthermore, the biochar in the biochar-basalt core-shell composite particles is a hydrogen peroxide-treated biochar intermediate obtained through the following steps: B1. Raw material: Take the aforementioned biochar; B2. Treatment: The biochar is mixed with an aqueous hydrogen peroxide solution, wherein the mass fraction of the aqueous hydrogen peroxide solution is 5 wt% to 30 wt% and the solid-liquid mass ratio is 1:5 to 1:30, and the mixture is treated at a temperature of 40°C to 80°C and under normal pressure for 0.5 h to 3 h. B3. pH control: Adjust the pH of the treatment system to 5.0 to 8.0, wherein the pH adjuster is selected from a 1.0 mol / L sodium hydroxide aqueous solution or a 1.0 mol / L hydrochloric acid aqueous solution; B4. Post-treatment: Filter and wash with deionized water until the pH of the washing solution is 6.0 to 8.0, and dry at 50°C to 105°C for 2 to 12 hours to obtain the hydrogen peroxide-treated biochar intermediate; B5. Quality control: The water content of the hydrogen peroxide-treated biochar intermediate shall not exceed 15 wt%.

[0009] Furthermore, the biochar is derived from biomass in agricultural and forestry waste and is prepared through the following steps: C1. Raw materials: Biomass from agricultural and forestry waste; C2. Drying: The biomass is dried at 60°C to 110°C for 2 to 12 hours; C3. Pyrolysis: The dried biomass is placed in a pyrolysis device and pyrolyzed under nitrogen protection and atmospheric pressure. The nitrogen protection is achieved by purging the pyrolysis device with nitrogen before heating until the oxygen volume fraction inside the device is no higher than 1 vol%. The oxygen volume fraction inside the device is maintained at no higher than 1 vol% during the pyrolysis and cooling processes. The temperature is increased to 450°C to 650°C at a heating rate of 3°C / min to 30°C / min and held at that temperature for 1 hour to 4 hours. C4. Cooling and pulverizing: After pyrolysis, cool to room temperature under nitrogen protection, pulverize, first pass through a 2.0 mm sieve to remove particles larger than 2.0 mm, then pass the undersize material through a 0.05 mm sieve to remove particles smaller than 0.05 mm, and collect the biochar with a particle size of 0.05 mm to 2.0 mm. C5. Quality control: The proportion of particles with a diameter of 0.05 mm to 2.0 mm in the biochar is 90 wt% to 100 wt%, based on the total dry weight of the biochar.

[0010] Furthermore, the biomass of the agricultural and forestry waste is selected from one or more of rice husks, straw, forestry pruning branches, or grapevines.

[0011] Furthermore, the alginate solids are obtained by calcium ion crosslinking and curing through the following steps: D1. Preparation of sodium alginate aqueous solution: Dissolve sodium alginate in deionized water to form sodium alginate aqueous solution, wherein the mass fraction of sodium alginate aqueous solution is 1wt% to 5wt%; the sodium alginate is dissolved by stirring at 10℃ to 40℃ for 10min to 120min. D2. Preparation of calcium chloride aqueous solution: Dissolve calcium chloride dihydrate in deionized water to form calcium chloride aqueous solution, wherein the mass fraction of the calcium chloride aqueous solution is 0.5wt% to 5wt% based on calcium chloride dihydrate; the calcium chloride dihydrate is dissolved by stirring at a temperature of 10℃ to 40℃ for 1 min to 30 min; D3. Crosslinking and curing: The wet pellet containing sodium alginate aqueous solution is placed into the calcium chloride aqueous solution and cured at a temperature of 10°C to 40°C and at normal pressure for 0.1 h to 2 h; the mass ratio of the wet pellet to the calcium chloride aqueous solution is 1:2 to 1:20; D4. Post-treatment: Remove the cured growth substrate spheres, wash with deionized water until the pH of the washing solution is 6.0 to 8.0, and dry at 40°C to 70°C for 2 to 24 hours; D5. Endpoint criterion: The moisture content of the growth substrate spheres after drying is not higher than 15 wt%.

[0012] Furthermore, the particle size of the soil fine particles is 0.15 mm to 2.0 mm, the particle size of the biochar is 0.05 mm to 2.0 mm, and the particle size of the growth substrate spheres is 4 mm to 25 mm. The solid components in the growth substrate spheres, by mass parts, are: 35 to 70 parts by mass of the soil fine particles, 10 to 35 parts by mass of the biochar, 5 to 25 parts by mass of the biochar-basalt core-shell composite particles, and 0.5 to 6 parts by mass of the alginate solids, calculated as sodium alginate. All mass parts are on a dry basis, where the dry basis is the mass obtained by drying the sample to constant weight at 105°C, and the constant weight is defined as a mass difference of no more than 0.1% between two consecutive weighings.

[0013] As a concept of this invention, the design of a quaternary composite of fine soil particles, biochar, biochar-basalt core-shell composite particles, and alginate gel is mainly used to enhance the comprehensive performance of the growth substrate sphere in terms of mechanical strength, pore function, ion slow release, and structural stability. The fine soil particles, as the skeleton component, provide interparticle contact points and packing structure, forming a porous network with a certain mechanical strength under the bonding of the alginate gel. Simultaneously, their own pores and surface active sites provide a basis for water retention and nutrient adsorption. The high specific surface area and well-developed microporous structure of biochar significantly improve the aeration and water retention capacity and nutrient adsorption capacity of the substrate sphere, while the heterogeneity of its hydrophobic-hydrophilic microregions facilitates water and air transport and root respiration. Biochar-basalt core-shell composite particles are constructed by anchoring the inorganic basalt shell to the biochar core surface through a tannic acid-ferric chloride coordination composite layer. Basalt, as a rigid inorganic mineral, significantly enhances the mechanical strength and wear resistance of the composite particles. Its rich silicates and metal oxides can also slowly release mineral elements required for plant growth. The coordination composite layer achieves a strong connection at the core-shell interface through the coordination bonds between polyphenols and metal ions, avoiding mechanical peeling of the shell. The outer silanization treatment further improves the interfacial compatibility between the core-shell composite particles and the alginate gel matrix. Alginate forms a three-dimensional network gel through calcium ion cross-linking. Its carboxyl functional groups provide abundant ion exchange sites, which can adsorb and slowly release nutrient ions such as nitrogen, phosphorus, and potassium, while also having a buffering capacity for heavy metal ions and salt ions. The calcium cross-linked gel network encapsulates and fixes soil fine particles, biochar, and core-shell composite particles, forming an integrated spherical structure, which improves the interfacial bonding strength between particles and the resistance to water erosion. The appropriate sodium alginate concentration and degree of cross-linking enable the gel network to maintain ion exchange function while having good dimensional stability, thus alleviating the contradiction between hydrophilic swelling and dry-wet stability.

[0014] This invention also discloses a method for preparing growth substrate spheres based on a biochar-soil composite structure, comprising the following steps: S1. Provide biochar; S2. Provide biochar-basalt core-shell composite particles: Obtain biochar-basalt core-shell composite particles; S3. Preparation of wet mixture: Fine soil particles, biochar, biochar-basalt core-shell composite particles are mixed with sodium alginate aqueous solution to obtain a wet mixture, wherein the mass fraction of the sodium alginate aqueous solution is 1 wt% to 5 wt% based on sodium alginate. S4. Spheroidization and calcium ion crosslinking curing: The wet mixture is spheroidized to obtain wet spheroid embryos. The wet spheroid embryos are then immersed in a calcium chloride aqueous solution for curing. The mass fraction of the calcium chloride aqueous solution is 0.5 wt% to 5 wt% based on calcium chloride dihydrate. The curing temperature is 10℃ to 40℃, the curing time is 0.1 h to 2 h, and the curing pressure is atmospheric pressure. The cured growth substrate spheres are then removed and washed with deionized water until the pH of the washing solution is 6.0 to 8.0 to obtain wet growth substrate spheres. S5. Drying: The wet growth substrate spheres are dried at a temperature of 40°C to 70°C for 2 hours to 24 hours to obtain the growth substrate spheres. The water content of the growth substrate spheres after drying is not higher than 15 wt%.

[0015] Furthermore, in step S3, the moisture content of the wet mixture is 10wt% to 40wt% based on the total mass of the wet mixture, and the mixing temperature is 10℃ to 40℃, and the mixing time is 5min to 60min.

[0016] Furthermore, the pelletizing method described in step S4 is selected from either extrusion spheronization or disc granulation.

[0017] Furthermore, the biochar-basalt core-shell composite particles obtained in step S2 are obtained by sieving, and their particle size is 50μm to 500μm.

[0018] Furthermore, the biochar-basalt core-shell composite particles also include a silanized layer located on the outer surface of the basalt shell, the silanized layer being formed of 3-aminopropyltriethoxysilane.

[0019] Furthermore, the core-shell structure of the biochar-basalt core-shell composite particles was characterized by scanning electron microscopy-energy dispersive spectroscopy (EDS) elemental distribution maps. The basalt shell layer was shown as a coating layer located outside the biochar core, which was confirmed by the enrichment and distribution of basalt characteristic elements in the peripheral region of the particles.

[0020] As another aspect of this invention, a five-step process of wet mixing, pelletizing, calcium cross-linking and curing, and drying is employed to enhance the process controllability and product quality consistency of the growth substrate pellets in terms of pelletizing processability, interfacial bonding strength, and dimensional stability. First, a wet mixture is prepared by mixing fine soil particles, biochar, biochar-basalt core-shell composite particles, and sodium alginate aqueous solution at suitable moisture content (10-40 wt%) and temperature (10-40 °C). The sodium alginate aqueous solution, acting as the mobile phase, imparts suitable rheological properties to the mixture, giving it good pelletizing processability and a wide process window, thus resolving the contradiction between the difficulty of pelletizing in low-viscosity systems and the poor flowability of high-viscosity systems. Second, uniformly sized wet pellets can be efficiently prepared through mechanical pelletizing methods such as extrusion spheronization or disc granulation. The requirements for the rheological properties of the wet mixture during mechanical pelletizing are far lower than those of traditional spray granulation or fluidized bed coating processes, significantly reducing sensitivity to raw material properties and process parameters. Next, the wet pellets are immersed in a calcium chloride aqueous solution for calcium ion crosslinking and curing. Calcium ions rapidly diffuse into the interior of the wet pellets and exchange with sodium alginate to form an insoluble calcium alginate gel network. This gel network encapsulates and fixes all solid components, significantly improving the interfacial bonding strength between particles and the overall structure's resistance to water erosion. The calcium crosslinking reaction is completed rapidly under mild conditions (10-40℃), avoiding the damage to the properties of biochar and alginate caused by high-temperature curing processes, and preserving the ion exchange function of alginate during the crosslinking process. Finally, excess moisture is removed through a mild drying process at 40-70℃. The suitable drying temperature avoids high-temperature degradation and excessive shrinkage of the alginate gel, ensuring that the moisture content of the final product is controlled below 15wt%, thus ensuring the product's storage stability and dimensional stability during use. Although the dried growth substrate pellets will absorb water and swell to some extent upon contact with water, the calcium-crosslinked gel network structure limits excessive expansion, maintaining good dimensional and structural stability during repeated wet-dry cycles, successfully achieving compatibility between pellet processing and service stability.

[0021] The synergistic effect of soil fine particles and biochar in the composite system is mainly reflected in the complementarity of framework support and pore regulation. Soil fine particles, as the main framework component (35-70 parts by mass), provide interparticle contact and stacking structure, giving the substrate sphere basic mechanical strength and dimensional stability. Biochar (10-35 parts by mass), as a functional pore component, fills the gaps between soil particles. Its high specific surface area and microporous structure significantly improve air permeability, water retention capacity and nutrient adsorption capacity. At the same time, its low bulk density avoids the decrease in pore connectivity caused by excessive densification, thus achieving a balance between structural strength and pore function. Compared with single biochar, biochar-basalt core-shell composite particles anchor a rigid inorganic basalt shell onto the surface of the biochar core through a tannic acid-ferric chloride coordination composite layer. The basalt shell significantly enhances the mechanical strength and wear resistance of the composite particles, while retaining the microporous structure and adsorption function of the biochar core. The strong interfacial connection of the coordination composite layer prevents shell peeling, and the outer silanization treatment further improves the interfacial compatibility with the gel matrix. The core-shell composite particles (5-25 parts by weight) serve as a multifunctional reinforcing component, combining strength, porosity, and slow release of mineral elements. The synergistic effect of the alginate gel network and all solid components is concentrated in the integration of interfacial bonding and ion exchange functions. The calcium-crosslinked alginate gel, as a continuous phase (0.5-6 parts by mass of sodium alginate), encapsulates and fixes fine soil particles, biochar, and core-shell composite particles, forming an integrated spherical structure. This significantly improves the interfacial bonding strength between particles and the resistance to water erosion. The carboxyl functional groups in the gel network provide abundant ion exchange sites, forming a synergistic nutrient release and ion buffering system with the surface functional groups of biochar and the slow release of mineral elements from basalt. The appropriate degree of crosslinking allows the gel network to maintain ion exchange function while possessing good dimensional stability, mitigating the conflict between hydrophilic swelling and wet / dry stability. Under simulated soil environmental conditions, the multi-level interfacial reinforcement structure constructed in this invention helps improve the stable existence potential of biochar components in the soil system.

[0022] Beneficial technical effects 1. Significantly Enhanced Mechanical Strength and Compressive Strength: Through the support of a fine soil particle skeleton, the rigidity enhancement of biochar-basalt core-shell composite particles, and the overall bonding of a calcium-crosslinked alginate gel network, the growth substrate spheres of this invention achieve a significant improvement in mechanical strength while maintaining sufficient porosity. The compressive strength ranges from 38.5±3.5N to 58.5±5.0N in Examples 1 to 4 (see Table 1), effectively withstanding mechanical stress during transportation, laying, and irrigation, preventing breakage and fine powder blockage of pores, extending service life, and reducing subsequent maintenance costs. Through the synergistic effect of the core-shell reinforced structure and the overall solidification of the calcium-crosslinked gel, the growth substrate spheres of this invention maintain structural integrity under hydraulic erosion and mechanical stress, helping to reduce fine powder loss.

[0023] 2. Optimized aeration and water retention capacity and pore connectivity: The high specific surface area and well-developed microporous structure of biochar, combined with the accumulated pores of fine soil particles, form a multi-scale pore network system, which significantly improves the aeration and water retention capacity of the substrate sphere, effectively improves the water and air balance of the rhizosphere microenvironment, and promotes root respiration and nutrient absorption. It is particularly suitable for application scenarios with strict requirements for aeration and water retention performance, such as saline-alkali land remediation and soilless cultivation in facility agriculture.

[0024] 3. Achieving a balance between ion exchange and slow-release function and size stability: The carboxyl functional groups of the calcium-crosslinked alginate gel network provide abundant ion exchange sites, which can effectively adsorb and slow-release nutrient ions such as nitrogen, phosphorus, and potassium, while having a buffering capacity for heavy metal ions and salt ions, reducing the toxic effects of saline-alkali soil and polluted soil on plant roots; the appropriate degree of calcium crosslinking allows the gel network to maintain ion exchange function while limiting excessive hydrophilic swelling, and it can still maintain good size stability in repeated wet-dry cycles, effectively solving the intrinsic conflict between the hydrophilic gel's exchange function and structural stability.

[0025] 4. Enhanced interfacial bonding strength and resistance to water erosion: The calcium cross-linked alginate gel network firmly encapsulates and fixes soil fine particles, biochar, and core-shell composite particles, forming an integrated spherical structure. The interfacial bonding strength is significantly improved, with a loss rate significantly lower than that of particles granulated with traditional organic binders. The strong interfacial connection of the tannic acid-ferric chloride coordination composite layer avoids the mechanical peeling of the basalt shell. The silanization treatment further improves the interfacial compatibility between the core-shell composite particles and the gel matrix, ensuring the structural integrity and performance stability of the substrate sphere during long-term service.

[0026] 5. Compatible with pelletizing processability and wide process window: Sodium alginate aqueous solution, as the mobile phase, imparts suitable rheological properties to the wet mixture, enabling it to have good pelletizing processability over a wide range of water content (10-40wt%) and temperature (10-40℃). It is suitable for various mechanical pelletizing processes such as extrusion spheronization and disc granulation, significantly reducing the sensitivity to raw material properties and process parameters, improving production process stability, and ensuring good product particle size uniformity (coefficient of variation less than 10%), making it suitable for industrial-scale production. The calcium crosslinking and curing reaction is completed rapidly under mild conditions, avoiding the damage to functional components caused by high-temperature curing processes, and reducing energy consumption and production costs. Attached Figure Description

[0027] Figure 1 This is a comparison diagram of XPS Fe2p high-resolution spectra of Example 1 and Comparative Example 7.

[0028] Figure 2 This is a comparison diagram of high-resolution XPSO1s coordinated oxygen in Example 1 and Comparative Example 7.

[0029] Figure 3 This is a comparison diagram of XPSC1s high-resolution tannin anchoring Example 1 and Comparative Example 7.

[0030] Figure 4 Comparison of Example 1 and Comparative Example 7 for FTIR spectral coordination layer O–H redshift Fe–O.

[0031] Figure 5 This is a comparison graph of XPSSi2p high-resolution spectroscopy silanization Example 1 and Comparative Example 8.

[0032] Figure 6 A comparison diagram of Example 1 and Comparative Example 8 for the high-resolution spectrum of XPSN1s amino groups.

[0033] Figure 7 This is a comparison diagram of the high-resolution spectrum of XPSO1s for Si–O–Si bonded oxygen in Example 1 and Comparative Example 8.

[0034] Figure 8 Comparison of XRD diffraction patterns at pyrolysis temperatures of 550℃ and 420℃ for Example 1 and Comparative Example 6.

[0035] Figure 9 This is a comparison graph of Raman spectral pyrolysis temperatures ID / IG between Example 1 and Comparative Example 6.

[0036] Figure 10 Macroscopic photograph of the growth substrate sphere prepared in Example 1.

[0037] Figure 11 This is a low-magnification scanning electron microscope image of the surface and cross-section of the substrate sphere grown in Example 1. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Example

[0039] The growth substrate spheres in this embodiment are spherical particles with a particle size of 12 mm. The growth substrate spheres in this embodiment comprise the following solid components, by dry weight: 53 parts by weight of fine soil particles, 23 parts by weight of biochar, 15 parts by weight of biochar-basalt core-shell composite particles, and 2.8 parts by weight of alginate solids (calculated as sodium alginate). The particle size of the fine soil particles in this embodiment is 0.8 mm, and the particle size of the biochar in this embodiment is 0.6 mm.

[0040] The biochar in this embodiment is derived from rice husks and is prepared through the following steps: Rice husks are selected and dried at 85°C for 6 hours; the dried rice husks are placed in a pyrolysis device and pyrolyzed under nitrogen protection and atmospheric pressure. Nitrogen protection in this embodiment is achieved by replacing the pyrolysis device with nitrogen before heating until the oxygen volume fraction inside the device does not exceed 1 vol%, and this oxygen volume fraction is maintained at no more than 1 vol% during the pyrolysis and cooling processes; the temperature is increased at 15°C / min. The temperature was raised to 550℃ at a heating rate and held for 2.5 hours. After pyrolysis, the mixture was cooled to room temperature under nitrogen protection. After pulverization, particles larger than 2.0 mm were first sieved through a 2.0 mm sieve, and particles smaller than 0.05 mm were sieved through a 0.05 mm sieve. Biochar with a particle size of 0.05 mm to 2.0 mm was collected. The mass percentage of particles with a particle size of 0.05 mm to 2.0 mm in the biochar of this embodiment was 95 wt%, based on the total dry weight of the biochar of this embodiment.

[0041] The biochar-basalt core-shell composite particles of this embodiment have a particle size of 200 μm and are prepared through the following steps: Take the biochar of this embodiment, mix it with an aqueous solution of hydrogen peroxide (15 wt% by mass, solid-liquid ratio 1:18), and treat it at 60°C and atmospheric pressure for 1.5 h; adjust the pH of the treatment system to 6.5 with a 1.0 mol / L sodium hydroxide aqueous solution; filter and wash with deionized water until the pH of the washing solution is 7.0, and dry at 75°C for 6 h to obtain the hydrogen peroxide-treated biochar intermediate; the water content of the hydrogen peroxide-treated biochar intermediate of this embodiment is 10 wt%.

[0042] The hydrogen peroxide-treated biochar intermediate of this embodiment was dispersed in an aqueous tannic acid solution with a mass concentration of 2.5 g / L. The solid-liquid mass ratio of the hydrogen peroxide-treated biochar intermediate to the aqueous tannic acid solution was 1:18. The mixture was stirred at 28°C for 30 min. Subsequently, an aqueous solution of ferric chloride hexahydrate was added with a concentration of 0.1 mol / L. The amount of ferric chloride hexahydrate added was based on a molar ratio of Fe³⁺ to tannic acid of 1:1, and the molar mass of tannic acid was calculated as 1701.2 g / mol. The mixture was stirred at 28°C for another 30 min to form biochar particles containing a coordination composite layer.

[0043] Basalt powder was added to the above system. In this embodiment, the mass ratio of basalt powder to biochar was 1:1. The mixture was stirred at 28°C for 60 min. The resulting solid was filtered, washed with deionized water, and dried at 65°C for 6 h. It was then dispersed in a mixed solvent of ethanol and water for silanization. A silane coupling agent, 3-aminopropyltriethoxysilane, was added to the silanization reaction system. The amount added was 2.5 wt% based on the dry weight of the resulting solid. The volume ratio of ethanol to water was 5:5. The ethanol used in this embodiment was anhydrous ethanol. The liquid-solid mass ratio of the silanization reaction system was... The ratio was 17:1, based on the dry weight of the obtained solid; the reaction pH was adjusted to 6.5 using a 1.0 mol / L hydrochloric acid aqueous solution, the reaction temperature was 35°C, the reaction time was 2 h, and the reaction pressure was atmospheric pressure; the mixture was filtered, washed with deionized water, and dried at 65°C for 6 h to obtain the biochar-basalt core-shell composite particles of this embodiment; the shell loading of the biochar-basalt core-shell composite particles of this embodiment was 22 wt%, which is the mass percentage of the basalt shell mass to the total dry weight of the biochar-basalt core-shell composite particles of this embodiment, and was determined by thermogravimetric analysis. The biochar-basalt core-shell composite particles of this embodiment also include a silanized layer located on the outer surface of the basalt shell, which is formed of 3-aminopropyltriethoxysilane.

[0044] The alginate solids of this embodiment are obtained by calcium ion crosslinking and solidification through the following steps: sodium alginate is dissolved in deionized water to form an aqueous sodium alginate solution, the mass fraction of which in this embodiment is 3 wt%; the sodium alginate is stirred at 25°C for 60 min to achieve dissolution; calcium chloride dihydrate is dissolved in deionized water to form an aqueous calcium chloride solution, the mass fraction of which in this embodiment is 2.5 wt% based on calcium chloride dihydrate; the calcium chloride dihydrate is dissolved at 25°C for 15 min.

[0045] The growth substrate spheres of this embodiment are prepared according to the following steps: Soil fine particles, biochar, and biochar-basalt core-shell composite particles of this embodiment are mixed with sodium alginate aqueous solution of this embodiment to obtain a wet mixture. The water content of the wet mixture is 25 wt% based on the total mass of the wet mixture, and the mixing temperature is 25°C for 30 min. The wet mixture is extruded and spheroidized to obtain wet sphere embryos. The wet sphere embryos are then immersed in calcium chloride aqueous solution of this embodiment for curing at 25°C for 1 h under normal pressure. The mass ratio of the wet sphere embryos to the calcium chloride aqueous solution is 1:10. The cured growth substrate spheres are removed and washed with deionized water until the pH of the washing solution is 7.0 to obtain wet growth substrate spheres. The wet growth substrate spheres are dried at 55°C for 12 h to obtain the growth substrate spheres of this embodiment. After drying, the water content of the growth substrate spheres of this embodiment is 8 wt%.

[0046] Features of this embodiment: This embodiment uses moderate parameter configurations, with moderate content of fine soil particles (53 parts by mass), moderate content of biochar (23 parts by mass), moderate content of core-shell composite particles (15 parts by mass), and moderate content of alginate solids (2.8 parts by mass); the biochar pyrolysis temperature is 550℃, and the holding time is 2.5h; the shell loading of the core-shell composite particles is 22wt%, the coordination layer construction adopts a Fe³⁺ to tannic acid molar ratio of 1:1, and a basalt to biochar mass ratio of 1:1; the sodium alginate solution mass fraction is 3wt%, the calcium chloride solution mass fraction is 2.5wt%, and the solidification time is 1h; the growth substrate sphere particle size is 12mm. The parameter combination of this embodiment has good stability and reproducibility, the process conditions are mild, and it is suitable for large-scale production, especially applicable to large-scale soil remediation projects, farmland improvement projects, and horticultural substrate production that require stable supply. Example

[0047] The growth substrate spheres in this embodiment are spherical particles with an aspect ratio of 1.3. The aspect ratio is the ratio of the maximum to the minimum diameter of the particles, and the particle size is 15 mm. The growth substrate spheres in this embodiment comprise the following solid components, on a dry basis: 58 parts by mass of fine soil particles, 18 parts by mass of biochar, 12 parts by mass of biochar-basalt core-shell composite particles, and 3.5 parts by mass of alginate solids (calculated as sodium alginate). The particle size of the fine soil particles in this embodiment is 1.2 mm, and the particle size of the biochar is 0.8 mm.

[0048] The biochar in this embodiment is derived from corn stalks and is prepared through the following steps: Corn stalks are selected and dried at 95°C for 4 hours; the dried corn stalks are placed in a pyrolysis device and pyrolyzed under nitrogen protection and atmospheric pressure. Nitrogen protection in this embodiment is achieved by replacing the pyrolysis device with nitrogen before heating until the oxygen volume fraction inside the device does not exceed 1 vol%, and this oxygen volume fraction is maintained at no more than 1 vol% during the pyrolysis and cooling processes; at 20... The temperature was increased to 580℃ at a heating rate of ℃ / min and held for 2 hours. After pyrolysis, the mixture was cooled to room temperature under nitrogen protection. After pulverization, particles larger than 2.0 mm were first sieved through a 2.0 mm sieve, and particles smaller than 0.05 mm were sieved through a 0.05 mm sieve. Biochar with a particle size of 0.05 mm to 2.0 mm was collected. The mass percentage of particles with a particle size of 0.05 mm to 2.0 mm in the biochar of this embodiment was 96 wt%, based on the total dry weight of the biochar of this embodiment.

[0049] The biochar-basalt core-shell composite particles of this embodiment have a particle size of 280 μm and are prepared through the following steps: Take the biochar of this embodiment, mix it with an aqueous solution of hydrogen peroxide (22 wt% by mass, solid-liquid ratio 1:12), and treat it at 70°C and atmospheric pressure for 1 h; adjust the pH of the treatment system to 7.0 with a 1.0 mol / L sodium hydroxide aqueous solution; filter and wash with deionized water until the pH of the washing solution is 7.2, and dry at 85°C for 4 h to obtain the hydrogen peroxide-treated biochar intermediate; the water content of the hydrogen peroxide-treated biochar intermediate of this embodiment is 8 wt%.

[0050] The hydrogen peroxide-treated biochar intermediate of this embodiment was dispersed in an aqueous tannic acid solution with a mass concentration of 3.5 g / L. The solid-liquid mass ratio of the hydrogen peroxide-treated biochar intermediate to the aqueous tannic acid solution was 1:12. The mixture was stirred at 35°C for 20 min. Subsequently, an aqueous solution of ferric chloride hexahydrate was added with a concentration of 0.15 mol / L. The amount of ferric chloride hexahydrate added was based on a molar ratio of Fe³⁺ to tannic acid of 1.5:1, and the molar mass of tannic acid was calculated as 1701.2 g / mol. The mixture was stirred at 35°C for another 20 min to form biochar particles containing a coordination composite layer.

[0051] Basalt powder was added to the above system. In this embodiment, the mass ratio of basalt powder to biochar was 1.5:1. The mixture was stirred at 35°C for 40 min. The resulting solid was filtered, washed with deionized water, and dried at 70°C for 4 h. It was then dispersed in a mixed solvent of ethanol and water for silanization. A silane coupling agent, 3-aminopropyltriethoxysilane, was added to the silanization reaction system. The amount added was 3.5 wt% based on the dry weight of the resulting solid. The volume ratio of ethanol to water in this embodiment was 7:3. The ethanol in this embodiment was 95 vol% ethanol. The liquid-to-solid mass ratio was 12:1, based on the dry weight of the obtained solid. The reaction pH was adjusted to 7.0 using a 1.0 mol / L sodium hydroxide aqueous solution, the reaction temperature was 42°C, the reaction time was 1.5 h, and the reaction pressure was atmospheric pressure. The mixture was filtered, washed with deionized water, and dried at 70°C for 4 h to obtain the biochar-basalt core-shell composite particles of this embodiment. The shell loading of the biochar-basalt core-shell composite particles of this embodiment was 30 wt%. The shell loading was defined as the percentage of the basalt shell mass to the total dry weight of the biochar-basalt core-shell composite particles of this embodiment, and was determined by ash content determination. The biochar-basalt core-shell composite particles of this embodiment also include a silanized layer on the outer surface of the basalt shell, which is formed of 3-aminopropyltriethoxysilane.

[0052] The alginate solids of this embodiment are obtained by calcium ion crosslinking and solidification through the following steps: sodium alginate is dissolved in deionized water to form an aqueous sodium alginate solution, the mass fraction of which in this embodiment is 4 wt%; the sodium alginate is stirred at 30°C for 40 min to achieve dissolution; calcium chloride dihydrate is dissolved in deionized water to form an aqueous calcium chloride solution, the mass fraction of which in this embodiment is 3.5 wt% based on calcium chloride dihydrate; the calcium chloride dihydrate is dissolved at 30°C for 10 min.

[0053] The growth substrate spheres of this embodiment are prepared according to the following steps: Soil fine particles, biochar, biochar-basalt core-shell composite particles, and sodium alginate aqueous solution are mixed to obtain a wet mixture. The moisture content of the wet mixture is 30 wt% based on the total mass of the wet mixture, and the mixing temperature is 30°C for 20 min. The wet mixture is then pelletized using a disc granulation method to obtain wet pellet embryos. These wet pellet embryos are then immersed in calcium chloride aqueous solution for curing at 30°C for 0.6 h under normal pressure. The mass ratio of the wet pellet embryos to the calcium chloride aqueous solution is 1:6. The cured growth substrate spheres are then removed and washed with deionized water until the pH of the washing solution reaches 6.8, resulting in wet growth substrate spheres. Finally, the wet growth substrate spheres are dried at 60°C for 8 h to obtain the growth substrate spheres of this embodiment. After drying, the moisture content of the growth substrate spheres of this embodiment is 10 wt%.

[0054] Features of this embodiment: This embodiment focuses on an efficient and rapid process route, with a high content of fine soil particles (58 parts by mass), a low content of biochar (18 parts by mass), a moderate content of core-shell composite particles (12 parts by mass), and a high content of alginate solids (3.5 parts by mass). The biochar pyrolysis adopts a high temperature of 580℃ and a fast heating rate of 20℃ / min, with a short holding time of 2h. The core-shell composite particles adopt a high shell loading of 30wt%, the coordination layer construction adopts a high Fe³⁺ to tannic acid molar ratio of 1.5:1, a basalt to biochar mass ratio of 1.5:1, and a high silane coupling agent dosage of 3.5wt%. The sodium alginate solution has a high mass fraction of 4wt%, the calcium chloride solution has a high mass fraction of 3.5wt%, and the solidification time is short at 0.6h. The growth substrate spheres have a particle size of 15mm. This embodiment features a highly efficient process, rapid response, and high shell loading capacity, making it suitable for applications requiring fast production cycles and high mechanical strength. It is particularly well-suited for engineering applications such as slope greening and mine restoration that require high-strength base spheres, as well as industrial production lines with high production efficiency requirements. Example

[0055] The growth substrate spheres in this embodiment are spherical particles with an aspect ratio of 1.2, which is the ratio of the maximum to the minimum diameter of the particles. The particle size is 8 mm. The growth substrate spheres in this embodiment comprise the following solid components, on a dry basis: 45 parts by mass of fine soil particles, 28 parts by mass of biochar, 18 parts by mass of biochar-basalt core-shell composite particles, and 1.5 parts by mass of alginate solids (calculated as sodium alginate). The particle size of the fine soil particles in this embodiment is 0.5 mm, and the particle size of the biochar is 0.3 mm.

[0056] The biochar in this embodiment is derived from forestry pruning branches and is prepared through the following steps: Forestry pruning branches are selected and dried at 75°C for 8 hours; the dried branches are placed in a pyrolysis device and pyrolyzed under nitrogen protection and atmospheric pressure. Nitrogen protection in this embodiment is achieved by purging the pyrolysis device with nitrogen before heating until the oxygen volume fraction inside the device does not exceed 1 vol%, and this oxygen volume fraction is maintained at no more than 1 vol% during the pyrolysis and cooling processes. The temperature was increased to 500℃ at a heating rate of 10℃ / min and held for 3 hours. After pyrolysis, the mixture was cooled to room temperature under nitrogen protection. After pulverization, particles larger than 2.0mm were first sieved through a 2.0mm sieve, and particles smaller than 0.05mm were sieved through a 0.05mm sieve. Biochar with a particle size of 0.05mm to 2.0mm was collected. The mass percentage of particles with a particle size of 0.05mm to 2.0mm in the biochar of this embodiment was 93wt%, based on the total dry weight of the biochar of this embodiment.

[0057] The biochar-basalt core-shell composite particles of this embodiment have a particle size of 120 μm and are prepared through the following steps: Take the biochar of this embodiment, mix it with an aqueous solution of hydrogen peroxide (10 wt% by mass) and the solid-liquid mass ratio is 1:22, and treat it at 50°C and normal pressure for 2 h; adjust the pH of the treatment system to 6.0 with a 1.0 mol / L sodium hydroxide aqueous solution; filter and wash with deionized water until the pH of the washing solution is 6.5, and dry at 60°C for 8 h to obtain the hydrogen peroxide-treated biochar intermediate; the water content of the hydrogen peroxide-treated biochar intermediate of this embodiment is 12 wt%.

[0058] The hydrogen peroxide-treated biochar intermediate of this embodiment was dispersed in an aqueous tannic acid solution with a mass concentration of 1.5 g / L. The solid-liquid mass ratio of the hydrogen peroxide-treated biochar intermediate to the aqueous tannic acid solution was 1:22. The mixture was stirred at 22°C for 45 min. Subsequently, an aqueous solution of ferric chloride hexahydrate was added with a concentration of 0.05 mol / L. The amount of ferric chloride hexahydrate added was based on a molar ratio of Fe³⁺ to tannic acid of 0.5:1, and the molar mass of tannic acid was calculated as 1701.2 g / mol. The mixture was stirred at 22°C for another 45 min to form biochar particles containing a coordination composite layer.

[0059] Basalt powder was added to the above system. In this embodiment, the mass ratio of basalt powder to biochar was 0.5:1. The mixture was stirred at 22°C for 90 min. The resulting solid was filtered, washed with deionized water, and dried at 60°C for 8 h. It was then dispersed in a mixed solvent of ethanol and water for silanization. A silane coupling agent, 3-aminopropyltriethoxysilane, was added to the silanization reaction system. The amount added was 1.5 wt% based on the dry weight of the resulting solid. The volume ratio of ethanol to water in this embodiment was 3:7. The ethanol used in this embodiment was anhydrous ethanol. The liquid-solid mass ratio of the silanization reaction system in this embodiment was... The ratio was 22:1, based on the dry weight of the obtained solid; the reaction pH was adjusted to 6.0 using a 1.0 mol / L sodium hydroxide aqueous solution, the reaction temperature was 28°C, the reaction time was 3 h, and the reaction pressure was atmospheric pressure; the mixture was filtered, washed with deionized water, and dried at 60°C for 8 h to obtain the biochar-basalt core-shell composite particles of this embodiment; the shell loading of the biochar-basalt core-shell composite particles of this embodiment was 12 wt%, which is the mass percentage of the basalt shell mass to the total dry weight of the biochar-basalt core-shell composite particles of this embodiment, and was determined by thermogravimetric analysis. The biochar-basalt core-shell composite particles of this embodiment also include a silanized layer located on the outer surface of the basalt shell, which is formed of 3-aminopropyltriethoxysilane.

[0060] The alginate solids of this embodiment are obtained by calcium ion crosslinking and solidification through the following steps: sodium alginate is dissolved in deionized water to form an aqueous sodium alginate solution, the mass fraction of which in this embodiment is 1.8 wt%; the sodium alginate is dissolved by stirring at 18°C ​​for 90 min; calcium chloride dihydrate is dissolved in deionized water to form an aqueous calcium chloride solution, the mass fraction of which in this embodiment is 1.5 wt% (calculated as calcium chloride dihydrate); the calcium chloride dihydrate is dissolved by stirring at 18°C ​​for 20 min.

[0061] The growth substrate spheres of this embodiment are prepared according to the following steps: Soil fine particles, biochar, biochar-basalt core-shell composite particles, and sodium alginate aqueous solution are mixed to obtain a wet mixture. The moisture content of the wet mixture is 18 wt% based on the total mass of the wet mixture, and the mixing temperature is 18°C ​​for 45 min. The wet mixture is extruded and spheroidized to obtain wet sphere embryos. These embryos are then immersed in calcium chloride aqueous solution for curing at 18°C ​​for 1.5 h under normal pressure. The mass ratio of the wet sphere embryos to the calcium chloride aqueous solution is 1:15. The cured growth substrate spheres are then removed and washed with deionized water until the pH of the washing solution reaches 6.5, resulting in wet growth substrate spheres. The wet growth substrate spheres are dried at 48°C for 18 h to obtain the growth substrate spheres of this embodiment. After drying, the moisture content of the growth substrate spheres of this embodiment is 11 wt%.

[0062] Features of this embodiment: This embodiment emphasizes a mild and slow process route, with a low content of fine soil particles (45 parts by mass), a high content of biochar (28 parts by mass), a high content of core-shell composite particles (18 parts by mass), and a low content of alginate solids (1.5 parts by mass); the biochar pyrolysis uses a low temperature of 500℃ and a slow heating rate of 10℃ / min, with a long holding time of 3h; the core-shell composite particles use a low shell loading of 12wt%, the coordination layer construction uses a low Fe³⁺ to tannic acid molar ratio of 0.5:1, a basalt to biochar mass ratio of 0.5:1, and a long reaction time of 3h; the sodium alginate solution mass fraction is low at 1.8wt%, the calcium chloride solution mass fraction is low at 1.5wt%, and the solidification time is long at 1.5h; the growth substrate spheres have a small particle size of 8mm. The process conditions in this embodiment are mild, the reaction is complete, and the biochar content is high. It is suitable for application scenarios with high requirements for pore structure and adsorption performance. It is particularly suitable for environmental remediation projects such as contaminated soil remediation, heavy metal adsorption, and organic pollutant degradation, as well as for fine horticulture and seedling substrate applications with high requirements for nutrient slow release and water retention performance. Example

[0063] The growth substrate spheres in this embodiment are spherical particles with an aspect ratio of 1.4, which is the ratio of the maximum to the minimum diameter of the particles. The particle size is 22 mm. The growth substrate spheres in this embodiment comprise the following solid components, on a dry basis: 64 parts by mass of fine soil particles, 12 parts by mass of biochar, 7 parts by mass of biochar-basalt core-shell composite particles, and 5.5 parts by mass of alginate solids (calculated as sodium alginate). The particle size of the fine soil particles in this embodiment is 1.8 mm, and the particle size of the biochar is 0.2 mm.

[0064] The biochar in this embodiment is derived from corn stalks and grapevines, and is prepared through the following steps: Corn stalks and grapevines are selected with a wet weight ratio of 1:1. The corn stalks and grapevines are dried at 65°C for 10 hours. The dried corn stalks and grapevines are placed in a pyrolysis device and pyrolyzed under nitrogen protection and atmospheric pressure. Nitrogen protection in this embodiment is achieved by replacing the pyrolysis device with nitrogen before heating until the oxygen volume fraction inside the device does not exceed 1 vol%. The oxygen content inside the device is maintained during the pyrolysis and cooling processes. The volume fraction is not higher than 1 vol%; the temperature is increased to 600℃ at a heating rate of 6℃ / min and held for 3.5h; after pyrolysis, the material is cooled to room temperature under nitrogen protection, pulverized, and then sieved through a 2.0mm sieve to remove particles larger than 2.0mm. The sieved material is then sieved through a 0.05mm sieve to remove particles smaller than 0.05mm. Biochar of this embodiment with a particle size of 0.05mm to 2.0mm is collected. The mass percentage of particles with a particle size of 0.05mm to 2.0mm in the biochar of this embodiment is 92wt%, based on the total dry weight of the biochar of this embodiment.

[0065] The biochar-basalt core-shell composite particles of this embodiment have a particle size of 80 μm and are prepared through the following steps: Take the biochar of this embodiment, mix it with an aqueous solution of hydrogen peroxide (8 wt% by mass) and the solid-liquid mass ratio is 1:8, and treat it at 75°C and atmospheric pressure for 2.5 h; adjust the pH of the treatment system to 7.5 with a 1.0 mol / L sodium hydroxide aqueous solution; filter and wash with deionized water until the pH of the washing solution is 7.5, and dry at 95°C for 3 h to obtain the hydrogen peroxide-treated biochar intermediate; the water content of the hydrogen peroxide-treated biochar intermediate of this embodiment is 6 wt%.

[0066] The hydrogen peroxide-treated biochar intermediate of this embodiment was dispersed in an aqueous tannic acid solution with a mass concentration of 0.5 g / L. The solid-liquid mass ratio of the hydrogen peroxide-treated biochar intermediate to the aqueous tannic acid solution was 1:8. The mixture was stirred at 18°C ​​for 50 min. Subsequently, an aqueous solution of ferric chloride hexahydrate with a concentration of 0.18 mol / L was added. The amount of ferric chloride hexahydrate added was based on a molar ratio of Fe³⁺ to tannic acid of 1.8:1, and the molar mass of tannic acid was calculated as 1701.2 g / mol. The mixture was stirred at 18°C ​​for another 50 min to form biochar particles containing a coordination composite layer.

[0067] Basalt powder was added to the above system. In this embodiment, the mass ratio of basalt powder to biochar was 0.2:1. The mixture was stirred at 18°C ​​for 100 min. The resulting solid was filtered, washed with deionized water, and dried at 75°C for 3 h. It was then dispersed in a mixed solvent of ethanol and water for silanization. A silane coupling agent, 3-aminopropyltriethoxysilane, was added to the silanization reaction system. The amount added was 4.2 wt% based on the dry weight of the resulting solid. The volume ratio of ethanol to water in this embodiment was 8:2. The ethanol used in this embodiment was anhydrous ethanol. The liquid-solid ratio of the silanization reaction system in this embodiment was... The mass ratio was 8:1, based on the dry basis mass of the obtained solid; the reaction pH was adjusted to 5.5 using a 1.0 mol / L hydrochloric acid aqueous solution, the reaction temperature was 45°C, the reaction time was 0.8 h, and the reaction pressure was atmospheric pressure; the mixture was filtered, washed with deionized water, and dried at 75°C for 3 h to obtain the biochar-basalt core-shell composite particles of this embodiment; the shell loading of the biochar-basalt core-shell composite particles of this embodiment was 8 wt%, which is the mass percentage of the basalt shell mass to the total dry basis mass of the biochar-basalt core-shell composite particles of this embodiment, and the shell loading was determined by ash content determination. The biochar-basalt core-shell composite particles of this embodiment also include a silanized layer located on the outer surface of the basalt shell, which is formed of 3-aminopropyltriethoxysilane.

[0068] The alginate solids of this embodiment are obtained by calcium ion crosslinking and solidification through the following steps: sodium alginate is dissolved in deionized water to form an aqueous sodium alginate solution, the mass fraction of which in this embodiment is 4.5 wt%; the sodium alginate is dissolved by stirring at 35°C for 25 min; calcium chloride dihydrate is dissolved in deionized water to form an aqueous calcium chloride solution, the mass fraction of which in this embodiment is 4.2 wt% (calculated as calcium chloride dihydrate); the calcium chloride dihydrate is dissolved by stirring at 35°C for 5 min.

[0069] The growth substrate spheres of this embodiment are prepared according to the following steps: Soil fine particles, biochar, and biochar-basalt core-shell composite particles of this embodiment are mixed with sodium alginate aqueous solution of this embodiment to obtain a wet mixture. The water content of the wet mixture is 35 wt% based on the total mass of the wet mixture, and the mixing temperature is 35°C for 12 min. The wet mixture is then pelletized into wet pellet embryos using a disc granulation method. These wet pellet embryos are then immersed in calcium chloride aqueous solution of this embodiment for curing at 35°C for 0.3 h under normal pressure. The mass ratio of the wet pellet embryos to the calcium chloride aqueous solution is 1:3. The cured growth substrate spheres are then removed and washed with deionized water until the pH of the washing solution reaches 7.8, resulting in wet growth substrate spheres. Finally, the wet growth substrate spheres are dried at 65°C for 5 h to obtain the growth substrate spheres of this embodiment. After drying, the water content of the growth substrate spheres of this embodiment is 9 wt%.

[0070] Features of this embodiment: This embodiment adopts a multi-parameter combination configuration, with high soil fine particle content (64 parts by mass), low biochar content (12 parts by mass), low core-shell composite particle content (7 parts by mass), and high alginate solids content (5.5 parts by mass); the biochar pyrolysis temperature is 600℃, the heat preservation time is relatively long (3.5h), and a relatively slow heating rate of 6℃ / min is adopted; the core-shell composite particles adopt a low shell loading amount of 8wt%, the coordination layer construction adopts a high Fe³⁺ to tannic acid molar ratio of 1.8:1 but a low basalt to biochar mass ratio of 0.2:1, a high silane coupling agent dosage of 4.2wt%, and a short silanization reaction time of 0.8h; the sodium alginate solution mass fraction is high of 4.5wt%, the calcium chloride solution mass fraction is high of 4.2wt%, and the solidification time is short of 0.3h; the growth substrate sphere particle size is large at 22mm, and the soil fine particle particle size is large at 1.8mm. This embodiment features high soil content, large particle size, and high binder content, forming a high-strength, large-particle structure. It is suitable for special application scenarios that require large-particle-size base spheres and high mechanical strength. It is particularly suitable for ecological restoration projects that require erosion resistance and wind erosion resistance, such as desertification control, sandy land greening, and riverbank protection, as well as engineering applications with special requirements for particle size and mechanical strength.

[0071] Comparative Example 1: It is basically the same as Example 1, except that the amount of fine soil particles is 32 parts by mass, while the amount of other components and preparation conditions remain unchanged.

[0072] Comparative Example 2: It is basically the same as Example 1, except that the amount of fine soil particles is 73 parts by mass, while the amounts of other components and preparation conditions remain unchanged.

[0073] Comparative Example 3: It is basically the same as Example 1, except that the amount of biochar is 8 parts by mass, while the amounts of other components and preparation conditions remain unchanged.

[0074] Comparative Example 4: It is basically the same as Example 1, except that the amount of biochar-basalt core-shell composite particles is 3 parts by mass, while the amount of other components and preparation conditions remain unchanged.

[0075] Comparative Example 5: It is basically the same as Example 1, except that the amount of alginate solids is 0.3 parts by mass based on sodium alginate, while the amounts of other components and preparation conditions remain unchanged.

[0076] Comparative Example 6: It is basically the same as Example 1, except that the biochar pyrolysis temperature is 420°C, while the amount of other components and preparation conditions remain unchanged.

[0077] Comparative Example 7: It is basically the same as Example 1, except that the coordination layer was not constructed during the preparation of biochar-basalt core-shell composite particles (step A1). Hydrogen peroxide-treated biochar intermediate was directly mixed with basalt powder. The mass ratio of basalt powder to biochar was 1:1. After stirring at 28°C for 60 min, subsequent silanization and post-treatment were carried out. The amount of other components and preparation conditions remained unchanged.

[0078] Comparative Example 8: It is basically the same as Example 1, except that the biochar-basalt core-shell composite particles were not silanized (step A3) during preparation. After obtaining the solid in step A2, the particles were filtered, washed with deionized water, and dried at 65°C for 6 hours to obtain the biochar-basalt core-shell composite particles. The amounts of other components and preparation conditions remained unchanged.

[0079] Performance testing: Compressive strength test: The test object is dried growth substrate sphere samples with a particle size of 4-25 mm. The purpose of the test is to evaluate the ability of the growth substrate spheres to resist breakage by external forces during transportation, application and use. This index is directly related to the integrity and functional maintenance of the substrate spheres in practical applications. The test principle is based on the mechanical response of the material under uniaxial compressive load. The compressive strength is characterized by measuring the maximum load-bearing capacity when the particles break. The experimental method uses a texture analyzer or a universal testing machine. A single growth substrate sphere is placed between a flat indenter and a compressive load is applied at a constant rate of 0.5 mm / min to 2 mm / min until the particles break. The maximum force value at the moment of breakage is recorded. Key parameters include test temperature 25±2℃, relative humidity 45-65%RH, compression rate 1 mm / min, and sample number n≥30. Data processing is expressed as mean ± standard deviation. The average compressive strength is calculated after removing outliers.

[0080] Air permeability test: The test object is the gas exchange capacity of the moist growth substrate ball in a simulated soil environment; the purpose of the test is to evaluate the supporting role of the internal pore structure of the growth substrate ball on root respiration and microbial activity. This index reflects the air permeability and water retention balance capacity of the substrate ball; the test principle is based on Darcy's law, and the air permeability coefficient is calculated by measuring the flow rate of gas through the substrate ball accumulation layer under a constant pressure difference; the experimental method is to fill the air permeability test device with the growth substrate balls, the accumulation height is 8-12cm, the accumulation density is natural, a constant air pressure difference of 10-50Pa is applied, and the gas flow rate after stabilization is measured; key parameters include test temperature 20±2℃, sample moisture content controlled at 60-80% of field capacity, air pressure difference 30Pa, and test duration 30min; data processing is to calculate the air permeability coefficient by gas flow rate and geometric parameters, the unit is cm / s, and the average value ± standard deviation of n≥3 parallel tests is taken.

[0081] Water retention performance test: The test object is the water retention characteristics of the growth substrate sphere after saturation and water absorption under natural water loss conditions; the test purpose is to evaluate the water retention capacity and slow-release water supply potential of the substrate sphere, which is related to the water supply stability of the plant root system; the test principle is based on the capillary water retention and adsorption water retention mechanism of the material, and the water retention performance is characterized by measuring the water loss rate and residual water content at different time points; the experimental method is to soak the growth substrate sphere in deionized water for 24h until saturation, and then place it in a constant temperature and humidity chamber at 25±2℃ and 50±5%RH. The weight is recorded every 2h for 48h; the key parameters include the initial saturated water content, the slope of the water loss curve, the water retention rate at 24h, and the residual water content at 48h; the data processing is to plot the water loss curve and calculate the water retention rate as (Wt-Wdry) / (W0-Wdry)×100%, where W0 is the saturated mass, Wt is the mass at time t, and Wdry is the dry weight mass, and the average value ± standard deviation of n≥5 samples is taken.

[0082] Water erosion resistance test: The test object is the structural stability and fine powder loss of the grown substrate spheres under water flow impact conditions; the test purpose is to evaluate the erosion resistance and interfacial bonding strength of the substrate spheres in rainfall, irrigation or river applications, which are directly related to the durability of the substrate spheres during service; the test principle is based on the particle erosion and disintegration mechanism in solid-liquid two-phase flow, and the water erosion resistance is characterized by measuring the mass loss rate and fine powder loss after constant water flow erosion; the experimental method is to place 50g of grown substrate spheres in the erosion test device, and at a flow rate of The sample was continuously flushed with a constant water flow of 0.5-2.0 m / s for 30 min, the flushing liquid was collected and passed through a 0.05 mm sieve to separate the fine powder, and then dried at 105℃ to constant weight. The standard reference was the relevant flushing test method in the soil and water conservation industry. Key parameters included water flow velocity of 1 m / s, flushing time of 30 min, and water temperature of 20±2℃. The fine powder loss rate was calculated as fine powder dry weight / initial dry weight × 100%, and the mass loss rate was (W0-W1) / W0 × 100%. The average value ± standard deviation of n≥3 parallel tests was taken.

[0083] Porosity and pore size distribution testing: The test object is the internal pore structure characteristics of the growth substrate spheres; the purpose of the test is to evaluate the microstructural basis of the substrate spheres' air permeability and water retention balance capacity. This index reflects the complex pore system formed by biochar, soil particles, and gel network; the test principle is based on gas adsorption or mercury intrusion porosimetry, calculating total porosity and pore size distribution through nitrogen adsorption-desorption isotherms or mercury intrusion curves; the experimental method uses a BET surface area and pore size analyzer, the sample pretreatment is vacuum degassing at 105℃ for 4 hours, and the test gas is high-temperature gas. Pure nitrogen (purity ≥99.999%) was used, the test temperature was liquid nitrogen temperature 77K, and the relative pressure range P / P0 was 0.01-0.99; the standard reference was GB / T21650.2 or ISO15901 series standards; key parameters included degassing temperature 105℃, degassing time 4h, and adsorbate nitrogen; data processing used the BET equation to calculate specific surface area, the BJH model to calculate pore size distribution, and statistical analysis of total pore volume, average pore size, and the proportion of micropores, mesopores, and macropores, taking the average value ± standard deviation of n≥3 samples.

[0084] Nutrient element slow-release performance test: The test object is the nutrient element release behavior of the growth substrate spheres under simulated use conditions; the test purpose is to evaluate the ion exchange slow-release function of the alginate gel network and the adsorption regulation effect of biochar, which reflects the continuous nutrient supply capacity of the substrate spheres; the test principle is based on ion exchange equilibrium and diffusion kinetics, and the slow-release characteristics are characterized by measuring the change of target element concentration in the soaking solution over time; the experimental method is to place 10g of growth substrate spheres in 500mL of deionized water and let them stand or stir slowly (50rpm) at 25±2℃. At time points of 5h, 1h, 2h, 4h, 8h, 12h, 24h, and 48h, 5mL samples were taken and replenished with an equal volume of fresh deionized water. The concentrations of Ca²⁺, K⁺, and Mg²⁺ ions were determined using inductively coupled plasma optical emission spectrometry (ICP-OES). The standard reference was the relevant testing standards for slow-release fertilizers. Key parameters included a liquid-to-solid ratio of 50:1, 8 sampling time points, and a test temperature of 25±2℃. The cumulative release curve was plotted, and the release rate and total release were calculated. A kinetic model was used for fitting, and the average value ± standard deviation of n≥3 parallel tests was taken.

[0085] Figure 1 The image shows a comparison of XPS Fe2p high-resolution spectra. X-ray photoelectron spectroscopy was used to characterize the surface chemical state of the biochar-basalt core-shell composite particles of Example 1 and Comparative Example 7. The basic parameters were the binding energy range of 700 to 740 eV and the intensity was normalized. The variable parameter was whether a tannic acid-Fe coordination composite layer was constructed. The results showed that Example 1 showed a clear Fe2p characteristic peak while the signal of Comparative Example 7 was extremely weak. This proves that the coordination composite layer can stably introduce and retain iron-related chemical states, thus providing verifiable chemical basis and reproducibility for interface modification.

[0086] Figure 2 This is a comparative diagram illustrating the high-resolution spectrum of coordinated oxygen in XPSO1s. X-ray photoelectron spectroscopy was used to characterize the chemical environment of oxygen-containing functional groups in the biochar-basalt core-shell composite particles of Example 1 and Comparative Example 7. The basic parameters are the binding energy range of 528 to 536 eV, with intensity normalization. The variable parameter is the change in the proportion of coordinated oxygen caused by the formation of tannic acid-Fe coordination structure. The results show that the O1s component related to coordination is enhanced in Example 1 on the high binding energy side, while it is weaker in Comparative Example 7. This proves that the introduction of a coordination layer can change the chemical environment of surface oxygen and provide stronger coordination binding characteristics, demonstrating the rationality of the scheme.

[0087] Figure 3 This is a comparative diagram illustrating the tannic acid anchoring in XPSC1s high-resolution spectra. X-ray photoelectron spectroscopy was used to characterize the carbon chemical bond composition of the biochar-basalt core-shell composite particles of Example 1 and Comparative Example 7. The basic parameters are the binding energy range of 280 to 292 eV and the intensity normalization display. The variable parameter is whether an organic layer rich in oxygen-containing carbon bonds is introduced on the surface of biochar through tannic acid. The results show that Example 1 has a more significant contribution in the C–O and O–C=O related regions compared to Comparative Example 7, while the main peak still maintains the carbon skeleton characteristics. This proves that tannic acid can effectively anchor on the surface without destroying the main carbon structure, thus supporting the correctness of the interface chemical design.

[0088] Figure 4 The image shows a comparison of the O–H redshift of the coordination layer to Fe–O in FTIR spectra. Fourier transform infrared spectroscopy was used to characterize the functional group vibrational features of the biochar-basalt core-shell composite particles in Example 1 and Comparative Example 7. The basic parameters were wavenumber ranges from 4000 to 400 cm⁻¹ and absorbance was normalized. The variable parameter was whether the tannic acid-Fe coordination layer was constructed, which led to changes in hydrogen bond and metal-oxygen bond characteristics. The results showed that the O–H stretching vibration peak position of Example 1 was redshifted relative to that of Comparative Example 7, and Fe–O related absorption appeared in the low wavenumber region, while it was not obvious in Comparative Example 7. This proves that coordination and metal-oxygen bond formation occur simultaneously, verifying the inherent consistency of the scheme at the molecular vibration level.

[0089] Figure 5 This is a comparative diagram illustrating the high-resolution silanization of XPSSi 2p. X-ray photoelectron spectroscopy was used to characterize the silicon chemical state of the biomass carbon-basalt core-shell composite particles of Example 1 and Comparative Example 8. The basic parameters are the binding energy range of 98 to 106 eV with normalized intensity. The variable parameters are whether the silane coupling reaction was completed and a more complete silicon-oxygen network was formed. The results show that the Si2p characteristic peak of Example 1 is stronger and more concentrated, while that of Comparative Example 8 is weaker. This proves that silanization can effectively introduce silicon-related structures on the surface, providing chemical support for subsequent interface stability and processability.

[0090] Figure 6 The diagram shows a comparison of the introduction of amino groups in XPSN 1s high-resolution spectra. The nitrogen-containing functional groups of the biochar-basalt core-shell composite particles of Example 1 and Comparative Example 8 were characterized by X-ray photoelectron spectroscopy. The basic parameters were the binding energy range of 394 to 406 eV and the intensity was normalized. The variable parameter was whether nitrogen-containing groups were introduced on the surface through an amino-containing silane coupling agent. The results showed that Example 1 showed a significant N1s signal while Comparative Example 8 was close to the background, proving that amino functionalization did indeed occur and there was detectable chemical evidence. This shows that the interface functional design is verifiable and reasonable.

[0091] Figure 7 The image shows a comparison of the high-resolution spectra of Si–O–Si bonded oxygen in XPSO1s. X-ray photoelectron spectroscopy was used to characterize the silicon-oxygen phase binding environment of the biochar-basalt core-shell composite particles in Example 1 and Comparative Example 8. The basic parameters are the binding energy range of 528 to 536 eV, with intensity normalization. The variable parameter is whether a higher proportion of Si–O–Si bonded oxygen is formed. The results show that the component of Si–O–Si bonded oxygen in Example 1 is enhanced in the corresponding high binding energy region, while that in Comparative Example 8 is weaker. This proves that the silicon-oxygen network is more fully constructed and can exist stably in the surface chemical layer, further supporting the rationality of the structural construction path of the scheme.

[0092] Figure 8 The image shows a comparison of XRD diffraction patterns at pyrolysis temperatures of 550℃ and 420℃. X-ray diffraction was used to characterize the crystal structure and microcrystal order of Example 1 and Comparative Example 6. The basic parameter was 2θ ranging from 10 to 50° with normalized intensity. The variable parameter was the change in microcrystal stacking and disorder of carbon materials caused by the difference in pyrolysis temperature. The results showed that the 002 diffraction peak of Example 1 was sharper at about 24° and the peak position changed visibly compared with Comparative Example 6. This proves that appropriately increasing the pyrolysis temperature helps to improve the order of carbon structure and form a more stable framework, providing a structural basis for the stability of material properties.

[0093] Figure 9 This is a comparison chart of the ID / IG ratio of the pyrolysis temperatures in Raman spectroscopy. Raman spectroscopy was used to characterize the degree of defects and graphitization in Example 1 and Comparative Example 6. The basic parameters were Raman shifts ranging from 1000 to 1800 cm⁻¹ with normalized intensity. The variable parameters were the relative intensity changes of the D and G peaks caused by different pyrolysis temperatures. The results showed that the ID / IG ratio of Example 1 was lower than that of Comparative Example 6, and the G peak was relatively more prominent. This proves that higher pyrolysis temperatures can reduce the defect ratio and improve the orderliness of the carbon structure. This is consistent with the XRD results, thus supporting the correctness and rationality of the scheme design as a whole.

[0094] Figure 10This is a macroscopic photograph of the growth substrate spheres prepared in Example 1. The sample was constructed based on a high-content biochar and soil mineral particle gradation and solidified using calcium alginate gel. The image shows that the particles are regularly spherical with a rough surface and a dark gray-black hue, demonstrating that the introduction of the high-carbon phase component successfully achieved the expected intrinsic light-absorbing appearance. Simultaneously, the multi-level particle stacking created a rich macroscopic porous surface structure, verifying the rationality of the substrate material composition design and molding process.

[0095] Figure 11 The image shows a low-magnification scanning electron microscope (SEM) image of the surface and cross-section of the growth substrate sphere in Example 1. The surface morphology reveals a rough, undulating surface composed of 0.8 mm soil particles and 0.6 mm biochar particles. The particles are cemented together in a dot-line pattern by a small amount of Ca-algite gel, forming a widely distributed open-cell structure. The cross-sectional morphology further reveals an internal two-phase support structure with coarse particles as the framework and a cementing phase as the connecting neck, resulting in an irregular, interconnected network of pores. This image demonstrates that the proposed method successfully constructed a porous substrate structure with a stable macroscopic framework and excellent pore connectivity.

[0096] As can be seen from the performance of the embodiments and comparative examples in Table 1, the embodiments exhibit a good overall balance in terms of compressive strength, air permeability, 24-hour water retention rate, fine powder loss rate, total porosity, and 24-hour cumulative release of Ca²⁺, demonstrating the superiority of the technical solution of this invention. Comparative Example 1, due to insufficient soil content, resulted in an inadequate particle skeleton, with a compressive strength of only 28.2 N and a fine powder loss rate as high as 8.5%, reflecting problems of loose structure and insufficient bonding. Comparative Example 2, due to excessive soil content, resulted in a porosity of 48.2%, a decrease in air permeability, and a reduction in water retention rate, demonstrating the negative impact of excessive densification on the balance of air permeability and water retention. Comparative Example 3, due to insufficient biochar content, resulted in a compressive strength of only 35.8 N; although the water retention rate was high, the nutrient slow-release capacity was insufficient. Comparative Example 4, due to insufficient core-shell composite particle content... The lack of sufficient alginate led to a decline in multiple properties; Comparative Example 5 had a compressive strength of only 25.8 N and a fine powder loss rate as high as 12.5% ​​due to excessively low alginate content, resulting in severely insufficient bonding strength; Comparative Example 6 had an imperfect biochar structure due to excessively low pyrolysis temperature, resulting in a decrease in compressive strength; Comparative Examples 7 and 8 had weakened interfacial bonding between the core-shell composite particles and the matrix due to the absence of coordination layer and silanization layer, respectively, resulting in a decrease in both compressive and erosion resistance, demonstrating the important role of coordination composite layer and silanization layer in improving interfacial bonding and overall mechanical properties.

[0097] Table 1. Performance comparison data between the examples and comparative examples. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A growth substrate sphere based on a biochar-soil composite structure, characterized in that, The growth substrate spheres are spherical or near-spherical particles, wherein the aspect ratio of the near-spherical particles is not greater than 1.5, and the aspect ratio is the ratio of the maximum diameter to the minimum diameter of the particles; the growth substrate spheres comprise the following solid components: Fine soil particles; Biochar; Biochar-basalt core-shell composite particles; Alginate solids, calculated as sodium alginate; The alginate solid is an alginate gel formed by calcium ion cross-linking and curing. The biochar-basalt core-shell composite particles include a biochar core, a basalt shell covering the outer surface of the biochar core, and a coordination composite layer located between the biochar core and the basalt shell. The coordination composite layer is formed by tannic acid and ferric chloride hexahydrate; The basalt shell loading amount is 5wt% to 40wt%, and the shell loading amount is the mass percentage of the basalt shell mass to the total dry mass of the core-shell composite particles.

2. The growth substrate sphere as described in claim 1, characterized in that, The biochar-basalt core-shell composite particles are prepared through the following steps: A1. Coordination Layer Construction: Hydrogen peroxide-treated biochar intermediates were dispersed in an aqueous tannic acid solution with a mass concentration of 0.1 g / L to 5 g / L and a solid-liquid mass ratio of 1:5 to 1:

30. The mixture was stirred at 15°C to 40°C for 5 to 60 minutes. Subsequently, an aqueous solution of ferric chloride hexahydrate with a concentration of 0.01 mol / L to 0.2 mol / L was added. The amount of ferric chloride hexahydrate added was based on a molar ratio of Fe³⁺ to tannic acid of 0.1:1 to 2:1 and a molar mass of tannic acid of 1701.2 g / mol. The mixture was stirred at 15°C to 40°C for another 5 to 60 minutes to form biochar particles containing a coordination composite layer. A2. Shell introduction: Basalt powder is added to the system obtained in step A1, wherein the mass ratio of basalt powder to biochar is 0.1:1 to 2:1, and the mixture is stirred at 15°C to 40°C for 10 min to 120 min. A3. Silanization: The solid obtained in step A2 is filtered, washed with deionized water, and dried at 50°C to 80°C for 2 to 12 hours. It is then dispersed in a mixed solvent of ethanol and water for silanization. A silane coupling agent, 3-aminopropyltriethoxysilane, is added to the silanization reaction system at an amount of 0.1 wt% to 5 wt% based on the dry weight of the solid obtained in step A2. The volume ratio of ethanol to water is 1:9 to 9:1, and the ethanol is anhydrous ethanol or ethanol with a volume fraction of not less than 95 vol%. The liquid-solid mass ratio of the silanization reaction system is 5:1 to 30:1 based on the dry weight of the solid obtained in step A2. The pH value is adjusted to 5.0 to 8.0 by dropwise addition of a 1.0 mol / L hydrochloric acid aqueous solution or a 1.0 mol / L sodium hydroxide aqueous solution. The reaction temperature is 20°C to 50°C, the reaction time is 0.5 to 4 hours, and the reaction pressure is atmospheric pressure. A4. Post-processing: Filtration, washing with deionized water, and drying at 50°C to 80°C for 2 to 12 hours to obtain the biochar-basalt core-shell composite particles; A5. Quality control: The shell loading of the biochar-basalt core-shell composite particles is 5wt% to 40wt%, and the shell loading is the mass percentage of the basalt shell mass to the total dry weight of the biochar-basalt core-shell composite particles. The shell loading is determined by thermogravimetric analysis or ash content determination.

3. The growth substrate sphere as described in claim 2, characterized in that, The biochar in the biochar-basalt core-shell composite particles is a hydrogen peroxide-treated biochar intermediate obtained through the following steps: B1. Raw material: Take the aforementioned biochar; B2. Treatment: The biochar is mixed with an aqueous hydrogen peroxide solution, wherein the mass fraction of the aqueous hydrogen peroxide solution is 5 wt% to 30 wt% and the solid-liquid mass ratio is 1:5 to 1:30, and the mixture is treated at a temperature of 40°C to 80°C and under normal pressure for 0.5 h to 3 h. B3. pH control: Adjust the pH of the treatment system to 5.0 to 8.0, wherein the pH adjuster is selected from a 1.0 mol / L sodium hydroxide aqueous solution or a 1.0 mol / L hydrochloric acid aqueous solution; B4. Post-treatment: Filter and wash with deionized water until the pH of the washing solution is 6.0 to 8.0, and dry at 50°C to 105°C for 2 to 12 hours to obtain the hydrogen peroxide-treated biochar intermediate; B5. Quality control: The water content of the hydrogen peroxide-treated biochar intermediate shall not exceed 15 wt%.

4. The growth substrate sphere as described in claim 1, characterized in that, The biochar is derived from biomass in agricultural and forestry waste and is prepared through the following steps: C1. Raw materials: Biomass from agricultural and forestry waste; C2. Drying: The biomass is dried at 60°C to 110°C for 2 to 12 hours; C3. Pyrolysis: The dried biomass is placed in a pyrolysis device and pyrolyzed under nitrogen protection and atmospheric pressure. The nitrogen protection is achieved by purging the pyrolysis device with nitrogen before heating until the oxygen volume fraction inside the device is no higher than 1 vol%. The oxygen volume fraction inside the device is maintained at no higher than 1 vol% during the pyrolysis and cooling processes. The temperature is increased to 450°C to 650°C at a heating rate of 3°C / min to 30°C / min and held at that temperature for 1 hour to 4 hours. C4. Cooling and pulverizing: After pyrolysis, cool to room temperature under nitrogen protection, pulverize, first pass through a 2.0 mm sieve to remove particles larger than 2.0 mm, then pass the undersize material through a 0.05 mm sieve to remove particles smaller than 0.05 mm, and collect the biochar with a particle size of 0.05 mm to 2.0 mm. C5. Quality control: The proportion of particles with a diameter of 0.05 mm to 2.0 mm in the biochar is 90 wt% to 100 wt%, based on the total dry weight of the biochar.

5. The growth substrate sphere as described in claim 1, characterized in that, The alginate solids were obtained by calcium ion crosslinking and curing via the following steps: D1. Preparation of sodium alginate aqueous solution: Dissolve sodium alginate in deionized water to form sodium alginate aqueous solution, wherein the mass fraction of sodium alginate aqueous solution is 1wt% to 5wt%; the sodium alginate is prepared by stirring at a temperature of 10℃ to 40℃ for 10min to 120min. D2. Preparation of calcium chloride aqueous solution: Dissolve calcium chloride dihydrate in deionized water to form calcium chloride aqueous solution, wherein the mass fraction of the calcium chloride aqueous solution is 0.5wt% to 5wt% based on calcium chloride dihydrate; the dissolution of calcium chloride dihydrate is achieved by stirring at a temperature of 10°C to 40°C for 1 min to 30 min; D3. Crosslinking and curing: The wet pellet containing sodium alginate aqueous solution is placed into the calcium chloride aqueous solution and cured at a temperature of 10°C to 40°C and at normal pressure for 0.1 h to 2 h; the mass ratio of the wet pellet to the calcium chloride aqueous solution is 1:2 to 1:20; D4. Post-treatment: Remove the cured growth substrate spheres, wash with deionized water until the pH of the washing solution is 6.0 to 8.0, and dry at 40°C to 70°C for 2 to 24 hours; D5. Endpoint criterion: The moisture content of the growth substrate spheres after drying is not higher than 15 wt%.

6. The growth substrate sphere as described in claim 1, characterized in that, The soil fine particles have a particle size of 0.15 mm to 2.0 mm, the biochar has a particle size of 0.05 mm to 2.0 mm, the core-shell composite particles have a particle size of 50 μm to 500 μm, and the growth substrate spheres have a particle size of 4 mm to 25 mm. The solid components in the growth substrate spheres, by mass parts, are: 35 to 70 parts by mass of the soil fine particles, 10 to 35 parts by mass of the biochar, 5 to 25 parts by mass of the biochar-basalt core-shell composite particles, and 0.5 to 6 parts by mass of the alginate solids, calculated as sodium alginate. All mass parts are on a dry basis, where the dry basis is the mass obtained by drying the sample to constant weight at 105°C, and constant weight is defined as a mass difference of no more than 0.1% between two consecutive weighings.

7. A method for preparing a growth substrate sphere based on a biochar-soil composite structure as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Provide biochar; S2. Provide biochar-basalt core-shell composite particles: Obtain biochar-basalt core-shell composite particles; S3. Preparation of wet mixture: Fine soil particles, biochar, biochar-basalt core-shell composite particles are mixed with sodium alginate aqueous solution to obtain a wet mixture, wherein the mass fraction of the sodium alginate aqueous solution is 1 wt% to 5 wt% based on sodium alginate. S4. Spheroidization and calcium ion crosslinking curing: The wet mixture is spheroidized to obtain wet spheroid embryos. The wet spheroid embryos are then immersed in a calcium chloride aqueous solution for curing. The mass fraction of the calcium chloride aqueous solution is 0.5 wt% to 5 wt% based on calcium chloride dihydrate. The curing temperature is 10℃ to 40℃, the curing time is 0.1 h to 2 h, and the curing pressure is atmospheric pressure. The cured growth substrate spheres are then removed and washed with deionized water until the pH of the washing solution is 6.0 to 8.0 to obtain wet growth substrate spheres. S5. Drying: The wet growth substrate spheres are dried at a temperature of 40°C to 70°C for 2 hours to 24 hours to obtain the growth substrate spheres. The water content of the growth substrate spheres after drying is not higher than 15 wt%.

8. The preparation method according to claim 7, characterized in that, In step S3, the moisture content of the wet mixture is 10wt% to 40wt% based on the total mass of the wet mixture, and the mixing temperature is 10℃ to 40℃, and the mixing time is 5min to 60min.

9. The preparation method according to claim 7, characterized in that, The pelletizing method described in step S4 is selected from either extrusion spheronization or disc granulation.

10. The preparation method according to claim 7, characterized in that, The biochar-basalt core-shell composite particles obtained in step S2 are obtained by sieving, and their particle size is 50μm to 500μm.

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